Trophic roles of heterotrophic nanoflagellates and ciliates among planktonic organisms in a hypereutrophic pond

We followed seasonal changes in abundance of bacteria, heterotrophic nanofl agellates (HNF), ciliates and crustaceans, and consumption of bacteria by t he protozoans, to investigate trophic interactions among these organisms in a hypereutrophic pond from March to October 1997. Densities of HNF and cil iates were high and attained a maximum of 1.4 x 10(5) and 3500 cells ml(-1) , respectively. However, the high densities decreased as chlorophyll concen tration increased. Since the predominant phytoplankton species was Microcys tis aeruginosa (Cyaplophyccae), toxin produced by the alga possibly affecte d growth of protozoans. Not only HNF but also ciliates were important consu mers of bacteria, and consumption of bacteria by ciliates varied at the sam e level as that of HNF from August to October. Bacterial turnover rate (% d (-1)) due to consumption by the protozoa ranged between 5.6 and 112 (mean 2 5), and there were significant relationships between densities of bacteria and specific ingestion rates (bacteria protozoan cell(-1) h(-1)) of the pro tozoans. These results suggest that the food linkage between bacteria and t he protozoans is substantial in the pond. We could not find any significant trophic relationships between HNF and ciliates, and between the protozoans and crustaceans.